1. Field of the Invention
The present invention relates to a solid-state imaging device and a method for manufacturing the same.
2. Description of Related Art
In recent years, solid-state imaging devices have been used for imaging devices of a digital still camera and a digital video camera and for image reading devices of a facsimile, a scanner and a copying machine, and the demands therefor have expanded. As major solid-state imaging devices, MOS-type solid-state imaging devices and CCD (charge-coupled device) type solid-state imaging devices are known. Recently, these solid-state imaging devices often are provided with color filters in order to capture a color image.
FIG. 5 is a partial cross-sectional view showing the configuration of a conventional solid-state imaging device. The solid-state imaging device of FIG. 5 is a CCD type solid-state imaging device. As shown in FIG. 5, the solid-state imaging device includes a plurality of photodiodes 22 that are arranged in a matrix in a semiconductor substrate 21. A vertical transfer unit 23 is provided at each column of the photodiodes 22 in the vertical direction.
In the example of FIG. 5, the semiconductor substrate 21 is a p-type silicon substrate, and the photodiodes 22 are n-type semiconductor regions. Each vertical transfer unit 23 includes a channel region 23b formed along a column of the photodiodes 22 in the vertical direction and a transfer electrode 23a provided on the channel region 23b. The transfer electrode 23a is covered with an insulation film 24.
Further, at an upper layer of each photodiode 22 an internal microlens 26 is formed via a first planarizing film 25. Further, at an upper layer of the internal microlens 26 color filters 28a to 28c are formed via a second planarizing film 27. The color filters 28a to 28c are formed corresponding to the photodiodes 22, respectively, and are arranged in a matrix so as to constitute a color filter array.
In the example of FIG. 5, the color filter 28a is a green (G) color filter, the color filter 28b is a blue (B) color filter and the color filter 28c is a red (R) color filter. One color filter corresponds to a light-receptive face of one photodiode 22, so that only one of red light, blue light and green light is incident on each photodiode 22.
Note here that, instead of such a primary-colors filter, a complementary-colors filter composed of the combination of cyan (C), magenta (MG), yellow (Y) and green (G) may be used. A method for forming the color filter 28a to 28c includes a staining method, a photoresist method or the like. A resist employed in the latter photoresist method includes a pigment dispersant resist, a dye dispersant resist or the like.
Further, at an upper layer of the color filter 28a to 28c a microlens 30 is formed via a third planarizing film 29, where a diameter of the microlens 30 is larger than that of the internal microlens 26. The microlens 30 also is formed corresponding to each photodiode 22, and is arranged in a matrix. In the solid-state imaging device of FIG. 5, the external light is gathered in two steps by means of the microlens 30 and the internal microlens 26, and then is incident on each photodiode 22. Therefore, the solid-state imaging device of FIG. 5 is devised so that the sensitivity to the light incident in a slanting direction can be improved.
Meanwhile, in the field of solid-state imaging devices, there is a tendency to increase the number of pixels year by year in order to improve their resolution. If the increase in the number of pixels results in an increase in the size of the solid-state imaging device, it becomes difficult to downsize a product that comes with such a solid-state imaging device. Therefore, it has been required to decrease the size of the pixels. It is expected that such a tendency will progress further in the future.
FIG. 6 is a partial cross-sectional view showing the configuration of a solid-state imaging device that is devised to increase the number of pixels and decrease the size of the pixels as compared with the example of FIG. 5. The configuration of the solid-state imaging device of FIG. 6 is similar to the configuration of FIG. 5 except that the number of pixels is increased and the size of the pixels is reduced. Since the pixels are decreased in size in the example of FIG. 6, the length H2 of one side of the photodiode 22 is shorter than the length H1 of one side of the photodiode of FIG. 5 (H2<H1).
However, since there is a limit to making the respective planarizing films and the color filter thinner, the length L2 between a photodiode 22 and a microlens 30 in the example of FIG. 6 becomes substantially equal to the length L1 between a photodiode 22 and a microlens 30 in the example of FIG. 5 (L2≈L1). Thus, the photodiode 22 of FIG. 6 will have a F value of the microlens 30 smaller than that of the photodiode of FIG. 5, thus degrading the sensitivity to the light incident in a slanting direction.
In order to cope with such a problem, JP 2002-314058 A for example proposes the formation of a color filter using a dry film formation method. As one example of the dry film formation method, an evaporation method is known, in which colored particles such as pigment particles or dye particles are deposited by evaporation at the formation region of the color filter. According to the dry film formation method, a color filter can be made thinner than in the conventional ones, so that the distance between a photodiode 22 and a lens 30 can be shortened, and therefore the above-stated problem can be solved.
However, in the case of a dry film formation method, a gap will be formed inside a color filter, thus causing problems of the degradation in a sensitivity and a S/N ratio of a solid-state imaging device. These problems will be described below with reference to FIG. 7.
FIG. 7 is a partial cross-sectional view of a conventional color filter formed by a dry film formation method. FIGS. 7A to C show examples where the deposition states of colored particles are different from each other. FIGS. 7A to C illustrate only the lines appealing in their cross section.
As shown in FIGS. 7A to C, the color filter 28a to 28c is an aggregation of a large number of colored particles 31, and in the case of the dry film formation method used, a gap 32 occurs between the deposited colored particles 31. Therefore, the light incident on the color filter 28a to 28c will be scattered at the gap 32. As a result, the sensitivity and the S/N ratio of the solid-state imaging device will be degraded as described above.